FM radio is in wide use in the field of radio broadcast. The term FM includes, for example, any of the Frequency Modulation methodologies used or developed for signal broadcasting in a frequency band assigned by the U.S. Federal Communications Commission (FCC), nominally in the transmission range 88 MHz to 108 MHz, which is near the middle of the Very-High-Frequency (VHF) television broadcast band. These Frequency Modulation technologies include both analog FM and digital FM.
The radio industry and the FCC have at present standardized on the iBiquity® IBOC (In-Band-On-Channel) hybrid analog-digital transmission system. This system permits FM stations in the U.S. to broadcast analog and digital signals simultaneously on their currently allocated channel frequency, if they use a single antenna to perform the simulcast.
At present, all U.S. FM radio transmission channels are 200 KHz wide, with standard analog FM broadcast modulation occupying only the center 100 KHz of the channel and with the IBOC signal using the outer 50 KHz on each side of the analog part of the channel. This characteristic of the IBOC signal imposes a need for sharp-cutoff filters to maintain signal separation, both between adjacent channels and between the analog and digital portions of the transmission on a single channel.
As an additional consideration, the FCC stipulates that the transmitted digital signal is to be 20 dB lower in signal strength than the analog signal. This may intrinsically place the digital transmitting antenna in a field as much as 10 times stronger than its own transmission.
One method of achieving an IBOC simulcast is to use two separate transmission systems feeding into two separate antennas on a single tower. Since the vertical position at which an antenna is mounted on a tower directly affects the antenna's achieved coverage, it would be desirable to collocate the analog and digital antennas not only on the same tower, but also at the same height above the ground. Further, since the azimuth pattern of an FM antenna is highly dependent on the interaction between the radiating device and the cross section of the tower structure, it would be desirable to mount both the analog and digital antennas in the same orientation to the tower.
When adding digital FM coverage to towers already in use for analog FM, a concern arises because many towers are full—that is, the towers have no additional aperture space available—so that some FM broadcasters may be required to interleave a second antenna within the aperture of their existing antenna. This introduces a challenge, because the analog and digital signals occupy the same segment of the frequency spectrum, yet are required to be isolated from each other. The current requirement for isolation between the IBOC digital signal and the analog signal is on the order of 35 dB. If the IBOC and analog antennas are to share the aperture, it is desirable to provide satisfactory isolation so that filtering requirements are kept within desirable ranges.
Accordingly, there is a need in the art for a method and apparatus to achieve isolation between separate in-channel FM antennas sharing common aperture space.